Illite, a common, layered silicate mineral, widely distributed in topsoil and subsoil, is utilized extensively in textiles, medicines, construction, and agriculture due to its distinctive chemical, physical, and mechanical properties. To understand the electronic structures and mechanical properties of illite better, they were studied under pressures ranging from 0 to 20 GPa using density functional theory (DFT) calculations. At ambient pressure, the calculated lattice constants and typical bond lengths were in good agreement with the experimental data. The lattice constants and volumes decreased with increasing pressure. While most bond lengths contracted under pressure, the O–H bond lengths remained nearly constant. Furthermore, analyses of the total and partial density of states, charge density distribution, and energy band structures revealed only minor variations under high pressure, implying that pressure exerts a limited influence on the electronic structure, and that illite retains its structural stability. The elastic constant, C33, was less than C11 and C22, whereas C66 exceeded C44 and C55 in the pressure range of 0–20 GPa. These results indicated that the (001) plane exhibits weaker resistance to deformation compared with the (010) and (100) planes, while its shear deformation resistance is greater. Also, Young’s modulus, the shear modulus, and the bulk modulus of illite increased with pressure, indicating enhanced resistance to elastic deformation and shear failure. The Poisson ratio also increased with pressure, indicating increased ductility. A decrease in the ratio of shear modulus to bulk modulus (G/B) under higher pressures indicated improved toughness. Moreover, the anisotropy of illite diminished with increasing pressure, implying a reduction in the directional dependence of its shear modulus, Young’s modulus, and bulk modulus. These theoretical calculations provide a microscopic understanding of the behavior of natural illite under pressure and offer valuable fundamental data for practical applications of illite.
